Certain devices such as wafer defect scanners, laser printers, document scanners, projectors and the like make use of a narrow collimated laser beam that usually scan across a flat surface along a straight line path. A typical optical scanning system for this purpose employs a tilting flat mirror to deflect the beam. The tilting micro-mirror serves as a central element in many Micro Electro Mechanical Systems (“MEMS”) devices and/or Micro Opto Electro Mechanical Systems (“MOEMS”) devices. For the convenience of the reader, these devices (i.e. MEMS and/or MOEMS) will be referred to hereinafter throughout the specification and claims as “MEMS” devices. Many of these MEMS devices comprise two types of electro-statically mirrors:                In-plane mirrors—this type of mirrors is usually driven at its resonance frequency, the stator and the rotor of these mirrors are at the same layer and the mirrors' driving pulse is usually of a rectangular type signal (these mirrors are also known as resonance mirrors).        Staggered mirrors—this type of mirrors is typically comprised of two different layers, one that comprises the stator while the other comprises the rotor. In some cases, where the stator or the rotor is tilted permanently after manufacturing, only one layer may be used for the stator and the rotor. The staggered mirrors may operate at their resonance frequency or at lower frequencies down to and including DC (these mirrors are also known as linear mirrors).        
In most MEMS devices, data that relate to the mirror's position must be very accurate. Unfortunately, the knowledge of the actuation voltage timing is usually insufficient to enable determining the accurate position. Temperature changes, laser power change and other factors contribute to an overall unacceptable error in many cases, e.g. where a MEMS mirror is used for projecting applications that require accuracy of much less than a pixel. Video standards, especially high resolution use hundreds of bits per scan line, hence the significance in the accuracy of the mirror's position.
There are several methods known in the art for monitoring the mirror's position, some of them rely on the electrostatic fingers to derive therefrom an indicator of the mirror position. Each position of the mirror is characterized by having different capacitance induced between the electrostatic fingers, thus, if the capacitance can be measured accurately, it may provide information regarding the mirror's position. The following attempts were made to harness the system capacitance for monitoring the mirror position:
US 2003174376 discloses a control system for controlling the position of the mirrors in a MEMS device. This disclosure uses the absolute capacitance of the mirror to detect the position of the mirror. According to the solution described in this publication, the MEMS mirror device is mounted on a substrate that includes a micro mirror that is movable pivotally about an axis, a first conductive layer on the mirror, a second conductive layer on the substrate, where the first and second conductive layers form a capacitor for determining the position of the mirror. The control according to this disclosure is of the absolute phase by means of measuring the absolute phase of the mirror. Unfortunately measuring this parameter in order to overcome the problem at hand has been found quite unsatisfactory for a number of reasons among which is the parasitic capacitance involved.
US 2008180516 discloses a MEMS mirror device that includes a CMOS substrate, a pair of electrodes, and a reflecting mirror moveable over the substrate and the electrodes. The voltages applied to the electrodes create an electrostatic force causing the edge of the mirror to be attracted to the substrate. According to this publication the position of the mirror can be detected and controlled by sensing a change in capacitance between the mirror edges, by detecting the differential capacitance between the two antipodal electrodes.
The above methods measure the absolute capacitance but still fail to provide reliable and accurate information regarding to the mirror position because of parasitic capacitance that is inherent to the system. In addition due to the nature of the electric circle that drives the mirrors, none of the above methods can measure the capacitance throughout the whole scanning cycle.